To match the lyrics of the songs to emotions, the researchers took data from labMT, a crowd-sourced website that rates the emotional valence of words (the degree to which they represent good or bad feelings). The details of when and where the songs originated from were taken from Gracenote, the same database as your music player probably uses to show artists’ information.
By correlating the valence of words with the type of chord accompanying them, the authors confirmed that major chords were associated more with positive words than minor chords. Unexpectedly, they found that seventh chords—chords with four different notes rather than the usual three—had an even higher association with positive words, even in the case of minor seventh chords. This is in constrast to other studies which have placed the valence of seventh chords between minor and major.